Science Exploration Notes Class 9 English Medium 2026-27

Science Exploration Chapter 2. Cell: The Building Block of Life

CBSE Notes New Syllabus: Class 9

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Chapter 2. Cell: The Building Block of Life

This section explains the remaining cell organelles, their structure and functions, and how all organelles work together to keep the cell alive. It also introduces the concept of cell division and concludes the chapter with important revision points.

19. Endoplasmic Reticulum (ER)

The Endoplasmic Reticulum is a network of membrane-bound tubes and sacs spread throughout the cytoplasm. It acts as the transport system of the cell and helps in the synthesis of important substances.

Definition

The Endoplasmic Reticulum (ER) is a membrane-bound network that transports materials within the cell.

Types of Endoplasmic Reticulum

• Rough Endoplasmic Reticulum (RER) – Ribosomes are attached to its surface. It mainly synthesises and transports proteins.
• Smooth Endoplasmic Reticulum (SER) – Ribosomes are absent. It synthesises fats (lipids) and helps in detoxifying harmful chemicals.

Functions

• Intracellular Transport – Carries materials from one part of the cell to another.
• Protein Synthesis – RER helps in the formation of proteins.
• Lipid Synthesis – SER manufactures fats and oils.
• Detoxification – SER removes harmful substances, especially in liver cells.

20. Ribosomes

Ribosomes are extremely small cell organelles found either attached to the Rough Endoplasmic Reticulum or freely floating in the cytoplasm. They are essential for the production of proteins.

Definition

Ribosomes are tiny, non-membrane-bound organelles where proteins are synthesised.

Functions

• Protein Synthesis – Manufacture proteins required for growth and repair.
• Cell Growth – Help in the formation of new cell components.
• Enzyme Production – Produce enzymes needed for metabolic activities.

Remember

Ribosomes are known as the "Protein Factories of the Cell."

21. Golgi Apparatus

The Golgi Apparatus receives proteins and lipids from the Endoplasmic Reticulum, modifies them and packs them for transport to different parts of the cell or outside the cell.

Definition

The Golgi Apparatus is a membrane-bound organelle responsible for packaging, modifying and transporting cellular materials.

Functions

• Modification – Modifies proteins and lipids.
• Packaging – Packs substances into small vesicles.
• Secretion – Helps release useful substances outside the cell.
• Lysosome Formation – Produces lysosomes.

22. Lysosomes

Lysosomes contain powerful digestive enzymes that break down waste materials, damaged organelles and foreign particles inside the cell.

Definition

Lysosomes are membrane-bound sacs containing digestive enzymes.

Functions

• Digest Cellular Waste – Remove damaged organelles.
• Destroy Foreign Particles – Protect the cell from harmful microorganisms.
• Recycle Materials – Reuse useful cellular components.
• Cell Cleanup – Keep the cell healthy by removing unnecessary substances.

Remember

Lysosomes are often called the "Suicidal Bags of the Cell" because they may digest their own cell when severely damaged.

23. Mitochondria

Mitochondria are the energy-producing organelles of the cell. They release energy from food through the process of cellular respiration.

Definition

Mitochondria are double membrane-bound organelles that produce energy in the form of ATP.

Functions

• Cellular Respiration – Break down food to release energy.
• ATP Production – Produce ATP required for all cellular activities.
• Support Cell Activities – Supply energy for growth, movement and repair.

Remember

Mitochondria are known as the "Powerhouse of the Cell."

24. Plastids

Plastids are membrane-bound organelles found only in plant cells. They are responsible for food preparation, storage and colouring of plant parts.

Definition

Plastids are specialised organelles present in plant cells.

Types of Plastids

• Chloroplast – Contains chlorophyll and performs photosynthesis.
• Chromoplast – Provides different colours to flowers and fruits.
• Leucoplast – Stores starch, proteins and oils.

Functions

• Photosynthesis – Preparation of food in green plants.
• Food Storage – Stores nutrients for future use.
• Colour Formation – Gives attractive colours to flowers and fruits.

25. Vacuoles

Vacuoles are membrane-bound sacs filled with cell sap. They store water, food, minerals and waste materials.

Definition

A vacuole is a membrane-bound storage sac present inside the cell.

Functions

• Storage – Stores food, water and minerals.
• Waste Collection – Stores waste products.
• Maintains Turgidity – Keeps plant cells firm.
• Water Balance – Helps regulate the water content of the cell.

Difference

26. Cell Division (Basic Idea)

As organisms grow, old cells are replaced by new cells through cell division. This process also helps in reproduction and healing of damaged tissues.

Definition

Cell division is the process by which one parent cell divides to form new daughter cells.

Importance

• Growth – Increases the number of cells.
• Repair – Replaces damaged and worn-out cells.
• Reproduction – Helps in the formation of new organisms.
• Continuity of Life – Transfers hereditary information to new cells.

27. ATP Education Chapter Summary

The cell is the basic structural and functional unit of life. Every cell contains specialised organelles that work together to perform essential life processes such as nutrition, respiration, transport, storage, growth and reproduction. The coordinated functioning of these organelles enables every living organism to survive, grow and reproduce.

ATP Education Final Revision Box

• Cell – Basic structural and functional unit of life.
• Cell Membrane – Selectively permeable protective covering.
• Cell Wall – Provides rigidity and protection in plant cells.
• Cytoplasm – Site of metabolic activities.
• Nucleus – Control centre containing DNA.
• Ribosomes – Protein factories.
• Golgi Apparatus – Packaging and secretion centre.
• Lysosomes – Digestive organelles.
• Mitochondria – Powerhouse of the cell.
• Chloroplast – Site of photosynthesis.
• Vacuole – Storage organelle.
• Cell Division – Essential for growth, repair and reproduction.

ATP Education Tip

For CBSE examinations, remember the function of every organelle with its nickname: Nucleus – Control Centre, Ribosomes – Protein Factory, Mitochondria – Powerhouse, Golgi Apparatus – Packaging Centre, Lysosomes – Suicidal Bags, Chloroplast – Kitchen of the Plant Cell. These are among the most frequently tested concepts in objective, competency-based and descriptive questions.